Vitamin D is strange among nutrients because it's barely a nutrient at all — it's a hormone precursor your skin makes from sunlight, and food is the backup plan rather than the main supply. That single fact explains almost everything confusing about it: why deficiency is so common at high latitudes and in winter, why "eat more of it" is weak advice, and why this is the one deficiency where supplementation is genuinely the default fix for a lot of people rather than a last resort. It also explains the hype — vitamin D got blamed for everything from cancer to depression, and the large trials that followed have mostly deflated those claims while leaving its real job, bone health, intact.
This is a single-nutrient deep dive. For how vitamin D sits alongside iron, B12, and magnesium, see our overview of the signs you might be low on the four common deficiencies. Here we go deep on D alone.
What vitamin D actually does
Vitamin D's core, well-established role is calcium and phosphate regulation. It dramatically increases the efficiency of calcium absorption in the gut, and without enough of it, the body can't mineralize bone properly. Severe, sustained deficiency causes rickets in children (soft, deforming bones) and osteomalacia in adults (soft, aching bones and muscle weakness). In older adults, low vitamin D is associated with more falls and fractures. Those bone and muscle effects are the parts of the vitamin D story that have held up.
The harder truth is that mild-to-moderate deficiency often causes no clear symptoms at all. People attribute fatigue and low mood to low vitamin D, but the evidence that supplementing fixes those symptoms in people who aren't deficient is weak. The big, broad-benefit claims have largely not survived rigorous testing — more on that below.
Sun vs. diet vs. supplement
Sun is the primary natural source. UVB radiation converts a cholesterol derivative in skin into vitamin D3. But the supply is unreliable: it depends on latitude, season, time of day, skin pigmentation (more melanin means less synthesis), age (older skin makes less), sunscreen use, and how much skin is exposed. Above roughly 37° latitude, winter sun is too weak to produce meaningful vitamin D for months at a time. And the obvious tension — more sun exposure means more skin cancer risk — means "just go outside more" is not a clean recommendation.
Diet is a weak source because few foods contain meaningful vitamin D naturally. Per the NIH Office of Dietary Supplements vitamin D fact sheet:
- Cooked salmon, 3 oz — about 570 IU
- Canned trout, 3 oz — similar
- Cod liver oil, 1 tsp — about 450 IU
- Fortified milk, 1 cup — about 120 IU
- Fortified cereals / orange juice — somewhat less
- One large egg yolk — about 44 IU
The RDA is 600 IU/day for adults up to 70, and 800 IU after 70. As the food list makes clear, hitting that from diet alone — without fatty fish most days — is genuinely hard, which is why fortification and supplements carry so much of the load.
Supplements are therefore the practical fix for many people, especially through winter at higher latitudes. D3 (cholecalciferol) is generally preferred over D2 for raising and maintaining blood levels.
Who's deficient
Vitamin D deficiency is among the most common worldwide. The higher-risk groups:
- People with limited sun exposure — indoor lifestyles, covered skin, housebound older adults.
- People with darker skin — more melanin means less cutaneous synthesis for the same sun.
- Older adults — skin synthesizes less with age, and many spend less time outdoors.
- People with obesity — vitamin D is fat-soluble and gets sequestered in adipose tissue, lowering circulating levels.
- People with fat-malabsorption conditions — Crohn's, celiac, cystic fibrosis, or after bariatric surgery, since vitamin D needs dietary fat to be absorbed.
- Breastfed infants — human milk is low in vitamin D, which is why supplementation is recommended for them.
- Anyone at higher latitudes through winter.
This is the one deficiency where "are you outdoors much?" is a genuinely useful screening question.
Testing: when it's worth it
The standard test is serum 25-hydroxyvitamin D [25(OH)D], which reflects total vitamin D from sun, food, and supplements. Interpretation isn't fully settled, but commonly used cutoffs put deficiency below about 12 ng/mL (30 nmol/L), inadequacy in the 12–20 ng/mL range, and adequacy at 20 ng/mL (50 nmol/L) and above.
The important nuance: most guideline bodies do not recommend routine population-wide screening of healthy, asymptomatic people. The NIH ODS and major task forces favor testing when there's a reason — bone pain or osteomalacia, osteoporosis, malabsorption, or several risk factors stacked together — rather than reflexively. If you have stacked risk factors and want certainty, a 25(OH)D test is cheap and worth it. If you're a healthy adult, many clinicians will reasonably just recommend a modest daily supplement through winter without testing first.
Dosing ranges
For maintenance, the RDA (600–800 IU/day) is the baseline target, and many adults take a daily supplement in the 800–2,000 IU range to stay in the adequate zone, particularly in winter. Treating a confirmed deficiency uses higher, time-limited doses under medical guidance. The key point for self-supplementers: you do not need megadoses to maintain adequacy. More is not better here, and there's a real ceiling.
The toxicity ceiling
Vitamin D is fat-soluble and stored in the body, so unlike most water-soluble vitamins, it can accumulate to harmful levels. The NIH ODS sets the Tolerable Upper Intake Level at 4,000 IU/day for adults.
The hazard of overdose is hypercalcemia — dangerously high blood calcium — which causes nausea, vomiting, weakness, frequent urination, and, if sustained, kidney stones, kidney damage, and calcification of soft tissues. According to the StatPearls review on vitamin D toxicity, toxicity is generally associated with 25(OH)D levels above 100 ng/mL (250 nmol/L) and is almost always caused by high-dose supplements or manufacturing errors — not by sun or food, because the skin self-regulates and natural foods don't contain enough. Documented poisonings have come from mislabeled products and from people taking enormous self-prescribed doses for months. The takeaway: stay near the recommended range, don't chase a number with megadoses, and treat the 4,000 IU UL as a real ceiling rather than a suggestion.
The hype, and what survived
Vitamin D was, for a decade, a candidate cure-all. The large VITAL trial — nearly 26,000 adults randomized to 2,000 IU/day of vitamin D3 or placebo and followed for over five years — found no reduction in invasive cancer or major cardiovascular events. Other large trials have similarly failed to show broad benefits of supplementation in people who weren't deficient. What remains solidly supported is the original job: correcting genuine deficiency to protect bone and muscle. So the honest framing is "fix a real shortfall," not "take it for everything."
See where you actually stand
Diet alone rarely covers vitamin D, which makes it the one nutrient where a label-reading food log is most likely to show you a persistent gap — and most likely to prompt a sensible, modest supplement rather than a guess. CalBurndown reads vitamin D off nutrition labels as you log meals and shows your intake as a percent of target, so you can see exactly how far food gets you (usually not far) before deciding whether sun or a supplement needs to close the rest. Our micronutrient calculator gives you an age-based target, and the guide to tracking micronutrients covers the mechanics. For why a calorie deficit makes vitamin D and bone health especially worth watching, see are you getting enough vitamins on a calorie deficit.
This is not medical advice. A symptom list can't diagnose vitamin D status, and high-dose supplements can cause real harm. Talk to a clinician about whether a 25(OH)D test or supplement is right for you.